Okay, let's break down this complex situation. You're dealing with a service entrance issue, which is always tricky due to the varying jurisdictions and codes involved.1. Ampacity Changes Over Time



Yes, ampacity values in the NEC have changed significantly over the past 50-60 years. Several factors influence this: Improved Insulation Technology

Modern insulation materials can withstand higher temperatures, leading to increased ampacity ratings for the same conductor size. Older insulation types like RH, RHW, etc., had lower temperature ratings than modern THHN, XHHW, etc.

Better Understanding of Heat Dissipation

Research and testing have led to a more refined understanding of how conductors dissipate heat in different installation scenarios (in conduit, direct burial, in free air, etc.).

Revised Ambient Temperature Corrections

Ambient temperature correction factors (how much you have to reduce ampacity based on the surrounding temperature) have also been adjusted over time.

Harmonic Considerations

With the increased use of non-linear loads (electronics, etc.), harmonic currents can significantly increase conductor heating. Modern ampacity calculations sometimes need to account for this.

2. Utility vs. Customer Responsibility



Generally, the point of demarcation (where utility responsibility ends and customer responsibility begins) is at the service point. The service point is typically defined as the connection point of the utility's equipment to the building's wiring. This is often the lugs in the meter enclosure. Based on your description, the most likely scenario is this



Duke Energy's responsibility likely ends at the transformer lugs. The wire from the transformer lugs through the junction box and to the meter mains is the customer's responsibility. Even though the utility initially installed the wire from the transformer lugs, it becomes the customer's responsibility. The utility likely provided the tap box so their employees could work on the transformer without killing the building. Important

Review your local utility's service rules and regulations. They will have a clear statement of the demarcation point. Call Duke Energy and ask for a clear definition of their responsibility point. They may have records of the original installation that can help clarify the ownership of the wiring.

3. The Original Installation (1965-1980)



A single 500 kcmil aluminum conductor serving a multi-family dwelling in that era could have been acceptable, depending on the code edition and the calculated load. However, it's very unlikely it was code-compliant to feed a 600A overcurrent protection device. The code requires overcurrent protection to protect conductors at their ampacity rating. NEC requirements for load calculations for multi-family dwellings were different then. Demand factors (the percentage of total connected load that is expected to be in use simultaneously) were often more generous than they are today. So, a smaller conductor might have been considered adequate. The original installation could have been grandfathered in under earlier code editions if it met the code requirements at the time of installation. However, when you make significant changes or upgrades, the "grandfathering" may be lost, and you'll have to comply with the current code.4. Your Problem



You're correct: a single 500 kcmil aluminum conductor rated at 310 amps cannot feed a 600 amp overcurrent device under current NEC. The parallel 500 kcmil conductors from the junction box to the breakers are appropriately sized for a 600 amp load, assuming the conditions of use allow for that ampacity. (This assumes you are using the 75C ampacity values for the cables)5. Recommended Actions


Confirm Ownership

Absolutely clarify with Duke Energy where their responsibility ends. Get it in writing if possible. Request any available documentation from them regarding the original service installation.2.

Load Calculation

Perform a thorough load calculation for the entire building according to the current NEC. This will determine the minimum service size you need. Consider future load growth too (electric vehicle charging, etc.).3.

Code Compliance

The NEC requires that you protect conductors at their ampacity rating and the overcurrent device should be rated no higher then the lowest ampacity of any piece of equipment on the circuit.4.

Feeder Replacement Options



If you absolutely cannot get parallel conductors from the transformer to the junction box due to the existing conduit

Consider using larger conductors if possible within the 3" conduit. This may require using compact stranded conductors or conductors with higher temperature ratings.

If the load calculation permits

You might be able to reduce the size of the overcurrent protection device in the meter mains (if possible/allowed) to a value that matches the ampacity of a larger single conductor in the existing 3" conduit. This is unlikely, but it's worth exploring.



Complete Service Upgrade

The best solution is likely a complete service upgrade. This will involve: Installing a new conduit run (potentially a larger size). Installing new conductors (properly sized for the calculated load and code requirements). Potentially upgrading the meter mains to accommodate the new service. This is the most expensive option, but it ensures compliance, provides a reliable service, and allows for future load growth.

Important Considerations



Permits

You will need permits for this work.

Licensed Electrician

This is a job for a qualified and licensed electrical contractor experienced in service entrance work.

Inspection

The installation will be inspected by the local electrical inspector.

Coordination

Coordinate closely with Duke Energy and your electrical contractor to ensure the new service meets all their requirements.

In summary, the most likely scenario is that the wiring from the transformer lugs through the junction box is customer owned. The mismatched ampacity of that wire and the 600A breaker is a code violation that you'll need to correct during the replacement.Consulting with a qualified electrical engineer and a licensed electrical contractor is highly recommended* to ensure a safe and compliant installation. Flag for review